1. Field of the Invention
The present invention generally relates to a wireless transmitter and receiver, and more particularly to a wireless transmitter and receiver using an ultra-wideband direct spread pulse communication system for transmitting and receiving two series of data through different spread pulse streams.
2. Description of the Related Art
An ultra-wideband wireless communication system has been developed as a new type of data communication system in a spread spectrum communication system. The ultra-wideband wireless communication system performs data communication by spreading data to an ultra wide frequency band of several GHz and overlapping the spread data with a pulse without using a carrier wave. An example of the ultra-wideband wireless communication system is disclosed in “Performance Evaluation of Internally Turbo-Coded Ultra Wideband-Impulse Radio (ITU-UWB-IR) System”, YAMAMOTO Yoshitake and OTSUKI Tomoaki, Technical Report of IEICE, pp. 25-30, RCS 2002-55 (2002-05). Because data transmitted with each frequency band has only intensity of noise magnitude in the ultra-wideband wireless communication system, wireless devices using the same frequency band interfere with each other and power consumption is low. Systems that utilize this ultra-wideband wireless communication system are disclosed in Japanese Patent No. 3564468 entitled “Ultra Wideband Wireless Transmitter and Receiver, and Ultra-Wideband wireless Communication Method” (hereinafter, referred to as “Patent Literature 1”) that corresponds to Japanese Patent Application No. 2002-262680, and U.S. Patent Application Publication No. 2004/0087291 A1 entitled “Ultra-Wideband Transmitted and receiver, and Ultra-Wideband Wireless Communication Method” (hereinafter, referred to as “Patent Literature 2”), which claims priority to Patent Literature 1, and which was published on May 6, 2004.
The ultra-wideband wireless communication system described in Patent Literature 1 will be described with reference to FIG. 12. FIG. 12 illustrates both a structure of a transmitter of FIG. 1 of Patent Literature 1 and a structure of a receiver of FIG. 7. In FIG. 12, a delay time controller 2 based on the data signal to be transmitted generates a monocycle pulse and then outputs signals K1˜K3 to matched filters 1-1˜1-3. An example of a structure of the matched filters 1-1˜1-3 is illustrated in FIG. 13. For example, K1 and K2 are output when digital data “0” is transmitted and K1 and K3 are output when digital data “1” is transmitted.
When the output signal K1 is transmitted from the delay time controller 2, the matched filter 1-1 outputs a reference pulse stream signal P1 for determination of data spread by a spreading code PN0. Further, the matched filter 1-2 receives the output signal K2 and then outputs a data pulse stream signal P2 spread by a spreading code PN1 later than the reference signal by a predetermined time T. Further, the matched filter 1-3 receives the output signal K3 and then outputs a data pulse stream signal P3 spread by the spreading code PN1 earlier than the reference signal by the predetermined time T. Herein, the spreading codes PN0 and PN1 are orthogonal to each other. After these signals are added by an adder 3 and are amplified by a power amplifier (PA) 4, a pulse stream signal P0 is radiated through an antenna 6.
In a receiver, a low noise amplifier (LNA) 7 amplifies a pulse stream signal P0 received through an antenna 5 and then outputs the amplified signal to matched filters 8-1 and 8-2. An example of the matched filters 8-1 and 8-2 is illustrated in FIG. 14. When the matched filter 8-1 mapped to a spreading code PN0 detects a reference signal, it outputs a correlation output signal S1. When the matched filter 8-2 mapped to a spreading code PN1 detects a data signal, it outputs a correlation output signal S2. A delay time measuring section 9 detects which one of the correlation output signals S1 and S2 is first input. Further, a data determining section 10 demodulates a data signal on the basis of a detection result of the delay time measuring section 9. In this case, data becomes “0” when the correlation output signal S2 is input later than the correlation output signal S1, and data becomes “1” when the correlation output signal S2 is input earlier than the correlation output signal S1.
When this technology is used, the ultra-wideband wireless communication system does not require a digital circuit for high-speed bit synchronization. Thus, a low-speed digital circuit with low power consumption can perform ultra-wideband wireless communication and can eliminate the multi-pass effect.
However, when a large amount of data, for example image signals, is transmitted using the above-described ultra-wideband direct spread pulse communication system, a data transmission rate per unit time needs to be high. To increase the transmission rate, an interval between pulse chips of a pulse stream to be transmitted should be narrowed, the number of pulse chips should be reduced, and a time length of a pulse stream P0 to be transmitted should be shortened.
However, when the number of chips is reduced to increase the transmission rate, the cross-correlation between the correlation outputs S1 and S2 of the matched filters 8-1 and 8-2 of the pulse stream P0 in the receiver is sufficiently reduced with respect to a spread pulse stream signal of the other side. Due to this influence, a desired/unwanted (D/U) ratio is degraded. Further, when 1-bit analog-to-digital (A/D) conversion is performed, a dynamic range is lowered and a bit error rate (BER) is degraded.
To clearly explain these problems, there will be described the well-known 7-chip Barker codes (e.g., PN0 of 1011000 and PN1 of 0001101) in which the number of chips is small and the auto-correlation characteristic is good. As illustrated in FIG. 15, in the 7-chip Barker codes, a D/U ratio of the auto-correlation characteristic is a large value of 7:1. However, when the 7-chip Barker codes are applied to the ultra-wideband direct spread pulse communication system as illustrated in FIG. 16, the cross-correlation characteristic between spreading codes PN0 and PN1 is bad and therefore the D/U ratio is reduced to a ratio up to 7:3 as illustrated in FIGS. 17 and 18. Because the components of FIG. 16 are substantially equal to those of FIG. 12 denoted by the same reference numerals, a repeated description is omitted. To improve the D/U ratio, the cross-correlation characteristic should be improved and a spreading code with an increased number of chips should be used. In this case, it is difficult for a high data transmission rate to be implemented.